A study of the dynamic behavior of super tankers anchored to the seabed, which function as exploration platforms for crude in the sea, is allowing for an improvement in their efficiency and will contribute to Petrobras hitting its production targets in deep waters over the next few years – up until 2005 the total production by the company in the country should hit 1.85 million barrels of crude oil per day, as against the current 1.1 million barrels in the middle of August. Begun in March of 1998 and with forecast to last four years, the project about anchoring systems is coordinated by Hernani Luiz Brinati, of the Naval and Oceanic Engineering Department of the Polytechnic School of the University of São Paulo (USP).
This is yet one more leadership effort by Petrobras in the technology of crude oil exploration in deep waters. An example of this technology domination is the Marlim Sul (South Marlin) field, in the Campos Basin, where crude oil is being extracted at a sea bed depth of 1,709 meters, a production world record in sea drills. The development of leading technology had a strong stimulus: of the proven 9.8 billion barrels of crude that make up the current reserves of crude oil and natural gas of the company, 75% are in deep waters (between 400 and1,000 meters) and ultra-deep (more than 1,000 meters).
The technical people believe that the growth in the production depends a lot on the successful exploration of these fields, where the floating systems – semi-submersible rigs and anchored supertankers – are the only viable forms of production in the sea. The Brazilian success in exploration in deep waters was only possible thanks to an intensive research program that began more than twenty years ago, with scientists and engineers from all over the country. One of the most recent studies is that which Brinati is coordinating with its focus on anchored supertankers or as they are known FPSOs – acronym forfloating production storage and offloading .
Single hulls
Originally the FPSOs were single hulled supertankers of up to 300,000 tons dead weight, which were transformed into petroleum production platforms. This is the case of the Vidal de Negreiros, some 320 meters in length and 240,000 tons in weight, built in the 70’s and which operated in the Campos Basin. At the beginning of the 90’s, these single hulled ships began to suffer restrictions in their sailing because of the risk that they represented to the environment – by current international regulation; tankers must have a double hull, more resistant and less subject to damage and leaks.
The solution for the shipping companies was to transform them into high seas production platforms, and today Petrobras is operating twelve FPSOs constructed during the 70’s. They have an advantage over the normal platforms: besides housing machinery for the separate production of oil and gas, they have a huge storage capacity.
“The objective of the thematic project was to attempt to find a better mastery of the behavior of the FPSO through different configurations of the systems of anchoring”, explains Brinati. When the exploration is carried out in shallow waters, up to 400 meters in depth, they can be installed like the structure of a fixed platform, an enormous metallic structure fixed to the sea bed. In the case of drills in deeper waters, however, the only alternative is to install floating systems, which are tied to the underwater sea bed by chains, made of steel or polyester.
Maritime forces
At this point the problems begin. The environmental forces incident upon the system of fastening in deep waters are very high: they include waves of up to15 meters, high-speed winds and currents of up to 2 meters per second. For this reason, the tying ropes of the floating system, that get as long as 3,000 meters in extension, move about considerably and run the risk of damage. At the same time, they can destabilize the platform or the ship, affecting production. One needs to take into account that by the proper conception of the project, the supertankers are naturally subject to movements of high amplitude.
“The main challenge of the oceanic engineers is to maintain the positioning of the vessels of large size in deep waters of more than 1,000 meters, in a safe manner and at low costs”, says Brinati. For this to occur, there are two anchorage systems in use. The more traditional, adopted by the oil industry throughout the world, is a turret system, which consists of an enormous vertical axle passing over the bow of the ship, from which come fastening cables and piping – in some cases there are more than forty pipes – in the direction of the deep sea. In this system the vessel can move itself freely around the turret, directing itself in accordance with the angle of incidence of the waves, of the wind and of the current.
However, the Dicas System – Abbreviated from differentiated compliance anchoring system -, created by Petrobras, comes from a method of conventional anchoring, with fastening cables around the ship. The distribution of the ties and of their tension, however, are critically planned in such a way that the vessel can oscillate within a determined amplitude. “The good part about this system is its cost, well below that of the turret. The main disadvantage is that it only permits to the FPSO an oscillation of steering (verified at the bow) of approximately 45 degrees, making it more susceptible to environmental factors”, explains Celso Pesce, of the Mechanical Engineering Department of the Polytechnic School, who is a part of Brinati’s group.
Tubes launching
Though the group’s researchers came into the project with high theoretical knowledge and the creation of non-linear mathematical models for the numerical simulation of the movements of the floating system, there have been other practical applications, reveals Pesce: “As well as the active participation in various studies referring to the projection of FPSOs, such as the future P-50 (Petrobras-50), at the beginning of this year, our team concluded a viability and dimensional study for the installation of a system of dynamic positioning in a tubes launching barge, the BGL-1.
Thanks to these research studies, which include the formation of hydrodynamic and aerodynamic models along with control, the BGL-1 will no longer be the type of barge tied to the platform, but will become dynamically positioned with its six motor propulsion system”. The BGL-1, which works in the construction and maintenance of modules on the upper decks of the platforms, serves to launch oil ducts onto the sea bed. Its precise positioning is essential for the success of the operation of pipe launching. Its motors, commanded by a control system, work automatically to correct the positioning and the direction, in such a manner that the piping is launched in the correct place.
With the traditional system of fastening, based on the launching of anchors and appropriate for lesser depths, the operation was very slow and complex. After having launched some hundreds of meters of piping, the barge had to be moved ahead to the front in order to continue the operation, which made the work expensive and slow, since it demanded the re-positioning of the anchors along the path by way of auxiliary barges, the “anchor towing handlers”.
Kazuo Hirata, of the Technology Research Institute commented: “This new operation, which makes the launching of the piping much more efficient, and allows for the expansion of the operation of the BGL-1 into deeper waters, was only possible thanks to the development of reliable computer models, created at the Polytechnic School and calibrated beginning with experiments done in the hydrodynamics laboratory of the IPT”.
Vortex studies
André Paiva Leite, a researcher with the Engineering and Production Department of Petrobras, reveals that the USP group’s work has been of considerable value to the company. “Independently from any direct result, these projects have the merit of maintaining Petrobras in contact with cutting edge of research”, he explains. “As well as this, the researchers are assisting in the formation of a specialized work force, essential for the development of national technology in offshore oil exploration.”
A second project, coordinated by José Augusto Penteado Aranha, of the Naval and Oceanic Engineering of the Polytechnic, is intimately linked to the first. It is looking to study one of the most complex phenomenon of classical mechanics: the vibrations induced by a vortex. “This phenomenon is still not well understood from the experimental point of view and of theoretical modeling”, explained Pesce, responsible for part of the experimentation. Vortices are whirling movements, such as whirlpools, which can occur both in the air (the whirlwind that forms on the wings of an aircraft, for example) and in water (the whirlpool that forms in the drain of a sink).
Intensive vibrations
In the exploration of offshore crude oil, vortices come about in the interaction of the maritime currents with the drilling pipes and production pipes (called risers) which come down from the platforms. When this occurs, cables and pipes vibrate due to the energy produced through the alternated emission by the vortices, and this generates tensions which can even cause a rupture of the material due to metal fatigue. “It is vital that the critical characteristics of a new structure that is subject to induced vibrations through vortices, be known at the initial phase of the project”, said Penteado Aranha.
The culmination of the project, concluded after five years of research, was the development of a method of analysis that would allow for an estimation of the useful life of pipes subject to maritime currents. “These systems of risers can reach costs as high as dozens of millions of dollars and their integrity is threatened by the intense vibrations generated by vortices”, says the researcher. “Consequently, it is important to understand the phenomenon, since it has a very high economic impact on off-shore petroleum exploration.”
To have an idea of the scope of the research – that includes both computerized studies of the dynamics of fluids, and experiments with reduced size models carried out in the experimentation tank of IPT, as well as simulations of the dynamic behavior of a pipe or riser -, one only needs to know that the structure of vortices is so complex that currently computers are only capable of numerically simulating a small part of them. “We have advanced in a steadily. We are very close to being able to simulate the phenomenon of induced vibrations by vortices on risers, with the inclusion of 3-dimensional effects on the draining effect”, points out Julio Meneghini, responsible for the fluid-dynamics computer program. “It was possible to create the LIFE – Laboratory of Interaction Fluid Structures – equipping it with work stations of the latest generation, of very high processing capacity”, adds Clóvis Martins, of the Mechanical Engineering sector.
Accurate understanding
“The project was essential for bringing together people from different departments who would work around a common theme. With this, we are aiming to reach a more accurate physical understanding of the phenomenon of the vortex, in an attempt to construct models capable of reproducing the phenomenon observed in laboratories”, said Penteado Aranha. “I believe that in two years we will reach this objective.”
In the two thematic projects, five IPT researchers, eleven from the Polytechnic and two from the Physics and Mathematics Institute were participants, as well as around twenty under-graduate and post-graduate students. There was also collaboration from scientists at the Imperial College in London and from the universities of Michigan and Cornell in the United States.
The academic production coming from the research included sixty papers presented at international conferences, fifteen articles published in scientific magazines abroad, as well as monographies, master’s dissertations and doctorate theses.
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